Reflectivity in Principled Shader?
Actually, it is the specular value input, specular is reflectivity. It's just that in some non PBR setups specular is a way of faking a reflected highlight without calulating actual reflections, and they called it specular. But in the principled shader specular means reflection and that's where you would put the reflection map, also sometimes called a specular map.

Play with the specular value and you'll see you can go from zero reflection to 100% mirror reflection. Roughness is basically your way of dulling down or polishing up the reflectivity, it's like a gloss value